refs: consolidate remote name validation

In preparation for a future patch, extract from remote.c a function that
validates possible remote names so that its rules can be used
consistently in other places.

Helped-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Sean Barag <sean@barag.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/refspec.c b/refspec.c
index f10ef28..98d1caa 100644
--- a/refspec.c
+++ b/refspec.c
@@ -201,6 +201,16 @@ int valid_fetch_refspec(const char *fetch_refspec_str)
 	return ret;
 }
 
+int valid_remote_name(const char *name)
+{
+	int result;
+	struct strbuf refspec = STRBUF_INIT;
+	strbuf_addf(&refspec, "refs/heads/test:refs/remotes/%s/test", name);
+	result = valid_fetch_refspec(refspec.buf);
+	strbuf_release(&refspec);
+	return result;
+}
+
 void refspec_ref_prefixes(const struct refspec *rs,
 			  struct strvec *ref_prefixes)
 {